This invention relates to a process for thermoforming reinforced polymer sheets and forming composite structures wherein at least one exterior surface exactly replicates the mold surface.
Thermoformed articles made of thermoplastic polymer sheets find application, among others, in the automotivie industry as well as in other industries. Because plastic offers considerable weight savings over metal, it has been increasingly used for making various auto parts that previously were stamped out of steel sheets. However, plastic body panels have not found wide acceptance, mainly because of their unsatisfactory appearance, which is due in part to defects caused by air trapped between the starting plastic sheet and the adjacent mold surface. When the sheet is very soft, it tends to collapse onto the mold surface before all the air can be removed, and the resulting air pockets cause imperfections on the sheet surface. This is especially so for shallow draw thermoforming. When the sheet is too hard, it cannot exactly replicate the mold surface and appears fairly rough. These defects are exaggerated when the thermoformed part is painted. See, for example, the article entitled "Detroit moves closer to plastic body panels" in the March, 1979, issue of Business Week, pp. 84F and 84G, the note entitled "Cosmetic repair of SMC produces Class A finish" on page 74 of the April, 1979, issued of Plastics World, and the discussion of surface quality on p. 74 of the November, 1980, issue of Plastics World. In all these texts it is suggested that surface imperfections can be corrected by subsequent coating.
A further problem is encountered when the thermoformed sheet is adhered to a backing sheet, which may serve as a reinforcing member and may be ribbed or corrugated for greater strength and rigidity. Frequently the points or areas of bonding of the backing sheet can be seen when looking at the outer face of the front sheet. This phenomenon, known as the "print through" or "read through", can be caused by the practice of effecting the bonding at a temperature at which both members are at or above their glass transition temperatures and are not crystallized. Print through can be avoided by use of a suitable adhesive, which can provide at moderate temperatures composites having good structural integrity.
It thus is desirable to be able to thermoform reinforced poly(ethylene terephthalate) (PET) sheets into articles which have at least one smooth surface, exactly replicating the mold surface, without imperfections, and to form composites which, in addition to having at least one such surface, do not suffer from the print through effect.